1. Field of the Invention
The invention generally relates to a method and apparatus for the preservation of living tissues, particularly organs.
2. Description of the Relevant Art
Although tissue transplantation and implantation have been viable since the 1960's, and have increased in popularity since that time, techniques for preservation of tissue have not become normalized. Initially, simple cold storage was used where the tissue was maintained in a cold, nonperfused preservation fluid. Perfused cold storage and hyperbaric cold perfused storage were subsequently shown experimentally to be superior to simple nonperfused cold storage. Since neither perfused cold storage nor hyperbaric cold perfused storage could be practically applied, nonperfused cold storage continued to be preferred. A disadvantage of nonperfused cold storage, however, is the limited period of viability of the tissue, typically due to significant oxygen decline in the storage medium resulting from the stored tissue's metabolic need for oxygen.
Because of the distance that often separates tissue donors and recipients; the portability of storage devices is of critical importance. In addition, the desire to increase the pool of tissue available for transplantation into any one recipient mandates that the storage time for the tissue be extended beyond that permitted with simple hypothermic storage, thus opening the possibility for a world-wide network of donors and recipients.
Pulsatile perfusion devices have been developed to sustain and extend the viability of extracorporeal living tissue for several hours pending the implant of the tissue. The advantage of pulsatile perfusion is that it mimics the natural state of the tissue by inducing flow through its arterial supply with oxygenated fluid, or perfusate. However, only limited success has been achieved with perfusion of tissue in the atmosphere (i.e., without submersing the perfused tissue in the perfusate). The danger of this method of perfusion is that a pressure gradient may develop across the capillary wall of the tissue, which is proportionate to the output of the perfusion pump. Under hypothermic conditions, perfusion pressures in excess of 20 mm Hg have resulted in capillary damage destroying and compromising the viability of the tissue being preserved.
Hypothermic pulsatile perfusion of tissue during storage can significantly extend storage time to 12-24 hours, without loss of tissue viability, due to reduced tissue metabolic rate and oxygen consumption. For example, cooling to 15 C reduces oxygen consumption of myocardial tissue to one-fifth of the rate at room temperature (e.g., 25 C). However, hypothermia alone is less protective than when it is combined with oxygenated perfusion, in that a continuous supply of oxygen is available in the latter case to support the metabolic oxygen requirements.
Hypothermic perfusion devices have been designed and are known in the art. However, devices that are currently available for hypothermic pulsatile perfusion are typically large, require significant volumes of compressed gas and electrical power, and/or also may necessitate an upright level orientation for operation. Additionally, these devices tend to be very complex, consisting of many intricate parts that must work precisely in concert.